Many treatments and procedures are carried out in the oil industry utilizing high viscosity fluids to accomplish a number of purposes. For example, in the oil industry, high viscosity aqueous well treating fluids or gels are utilized in treatments to increase the recovery of hydrocarbons from subterranean formations, such as by creating fractures in the formation. High viscosity aqueous fluids are also commonly utilized in well completion procedures. For example, during the completion of a well, a high viscosity aqueous completion fluid having a high density is introduced into the well to maintain hydrostatic pressure on the formation which is higher than the pressure exerted by the fluids contained in the formation, thereby preventing the formation fluids from flowing into the wellbore. High viscosity treating fluids, such as fracturing gels, are normally made using dry gel additives or agents which are mixed with water or other aqueous fluids at the job site. Such mixing procedures have some inherent problems, particularly on remote sites or when large volumes are required. For example, special equipment for mixing the dry additives with water is required, and problems such as chemical dusting, uneven mixing, and lumping result. The lumping of gels occurs because the initial contact of the gel with water results in a very rapid hydration of the outer layer of particles which creates a sticky, rubbery exterior layer that prevents the interior particles from contacting water. The net effect is formation of what are referred to as “gel balls” or “fish eyes”. These hamper efficiency by lowering the viscosity achieved per pound of gelling agent and also by creating insoluble particles that can restrict flow both into the well formation and back out of it. Thus, simply mixing the untreated gel directly with water is not a very successful method of preparing a smooth homogeneous gel free from lumps.
A method directed to solving this problem is to control particle size and provide surface treatment modifications to the gel. It is desired to delay hydration long enough for the individual gel particles to disperse and become surrounded by water so that no dry particles are trapped inside a gelled coating. This can be achieved by coating the gel with materials such as borate salts, glyoxal, non-lumping HEC, sulfosuccinate, metallic soaps, surfactants, or other materials of opposite surface charge to the gel. A stabilized gel slurry (SPS), also referred to as a liquid gel concentrate (LGC), is the most common way to improve the efficiency of a gel addition to water and derive the maximum yield from the gel. The liquid gel concentrate is premixed and then later added to the water. In U.S. Pat. No. 4,336,145 to Briscoe, assigned to the assignee of the present invention and incorporated herein for all purposes, a liquid gel concentrate is disclosed comprising water, the gel, and an inhibitor having the property of reversibly reacting with the hydratable gel in a manner wherein the rate of hydration of the gel is retarded. Upon a change in the pH condition of the concentrate such as by dilution or the addition of a buffering agent to the concentrate, upon increasing the temperature of the concentrate, or upon a change of other selected condition of the concentrate, the inhibition reaction is reversed, and the gel or gels hydrate to yield the desired viscosified fluid. This reversal of the inhibition of the hydration of the gelling agent in the concentrate may be carried out directly in the concentrate or later when the concentrate is combined with additional water. The aqueous-based liquid gel concentrate of Briscoe has worked well at eliminating gel balls and is still in routine use in the industry. However, aqueous concentrates can suspend only a limited quantity of gel due to the physical swelling and viscosification that occurs in a water-based medium. Typically about 0.8 pounds of gel can be suspended per gallon of the concentrate.
To solve this problem, a hydrocarbon carrier fluid is used, rather than water, so higher quantities of solids can be suspended. For example, up to about five pounds per gallon of gel may be suspended in a diesel fuel carrier. Such a liquid gel concentrate is disclosed in U.S. Pat. No. 4,722,646 to Harms and Norman, assigned to the assignee of the present invention and incorporated herein for all purposes. Such hydrocarbon-based liquid gel concentrates work well but require a suspension agent such as an organophylic clay or certain polyacrylate agents. The hydrocarbon-based liquid gel concentrate is later mixed with water in a manner similar to that for aqueous-based liquid gel concentrates to yield a viscosified fluid, but hydrocarbon-based concentrates have the advantage of holding more gel.
A problem with prior methods using liquid gel concentrates occurs in offshore situations. The service vessels utilized to supply the offshore locations have a limited storage capacity and must, therefore, often return to port for more concentrate before they are able to do additional jobs, even when the liquid gel concentrate is hydrocarbon-based. Therefore, it would be desirable to be able to mix a well treatment gel on-demand during the treatment of the subterranean formation from dry ingredients. For example, such an on-line system could satisfy the fluid flow requirements for large hydraulic fracturing jobs during the fracturing of the subterranean formation by mixing the fracturing gel on demand.
One method and system for on-demand mixing of a fracturing gel is disclosed in U.S. Pat. No. 4,828,034 to Constien et al., herein incorporated by reference, in which a fracturing fluid slurry concentrate is mixed through a static mixer device on a real-time basis to produce a fully hydrated fracturing fluid during the fracturing operation. This process utilizes a hydrophobic solvent, which is characterized by a hydrocarbon such as diesel as in the hydrocarbon-based liquid gel concentrates described above. Such a slurry concentrate typically involves a gel slurry wherein a hydratable gel is dispersed in a hydrophobic solvent in combination with a suspension agent and a surfactant with or without other optional additives commonly employed in well treatment applications. Because of the inherent dispersion of the hydratable gel in the oil-based fluid (i.e., lack of affinity for each other), such fracturing fluid slurry concentrates tend to eliminate lumping and premature gelation problems and tend to optimize initial dispersion when added to water. However, most recently, there have been some problems with hydrocarbon-based liquid gel concentrates because some well operators object to the presence of these fluids, such as diesel, even though the hydrocarbon represents a relatively small amount of the total fracturing gel once mixed with water. And, there are environmental problems associated with the clean-up and disposal of well treatment gels containing hydrocarbons. Also, diesel, surfactants, suspension agents and other additives increase the cost of the well treatment fluid, not to mention the cost to transport these materials to and from the well site. These hydrocarbon-related problems would also apply to the process of Constien.
Another problem associated with some prior art methods for hydrating gels is that the gelling agent must subsequently be mixed in holding tanks for a considerable length of time for hydration of the gelling agent to occur, especially in the use of water-based fracturing fluids including a gelled and cross-linked polysacharade gelling agent.
Accordingly, there is a need for an on-demand process to eliminate the environmental problems and objections related to hydrocarbon-based concentrates and provide for more efficient methods whereby the treating fluids do not have to employ hydrocarbon-based concentrates such as LGCs to prepare treating fluids.
U.S. Pat. No. 5,190,374, to Harms et al., which is incorporated herein by reference thereto for purposes of disclosure, assigned to the assignee of the present invention, discloses method and apparatus for substantially continuously producing a fracturing gel, without the use of hydrocarbons or suspension agents, by feeding the dry polymer into an axial flow mixer which uses a high mixing energy to wet the polymer during its initial contact with water. After initial mixing, the additional water may be added to the mixer to increase the volume of water-polymer slurry produced thereby. In Harms, a predetermined quantity of hydratable polymer in a substantially particulate form is provided to a polymer or solids inlet of a water spraying mixer. A stream of water is supplied to a water inlet of the mixer, and the water and polymer are mixed in the mixer to form a water-polymer mix prior to discharge from the mixer. The mixer is preferably mounted adjacent to the upper portion of a mixing or primary tank, and an agitator may be provided in the mixing tank to further agitate and stir the slurry. The slurry may be transferred from the mixing tank to a holding or secondary tank after which it is discharged to the fracturing process. A high shear device may be disposed in the holding tank. A pump may be used for transferring the slurry from the mixing tank to the holding tank.
Although Harms discloses an on-line mixing system which may be used with untreated and uncoated polymers, in practice there are problems with the Harms mixing system. For example, the powder splatters inside the mixer, sticks to the walls of the mixer, and builds up, eventually choking flow through the mixer. The sequential opening of the water orifices in sets of six orifices inadequately wets the powder at low flow rates, and allows unwetted powder to pass. Another problem is created by the entrainment of air in the fluid mixed in the mixer which impairs the ability of the pump to adequately pump the mixture from the mixer. Another problem is the creation of additional discharge of the pump into the holding tank. The entrained air compels the use of deaerating chemicals with the system. Another problem is the lack of a controlled flow path and, therefore, the hydration time in the holding tank, i.e., the hydrating slurry can create unpredictable flow channels through the tank which cause non-uniform residence times of portions of the slurry in the tank. Another problem is the large lag time (5–10 minutes) involved in changing the viscosity of the gel discharged from the holding tank, i.e., the only way to alter the viscosity of the gel is to change the powder/water ratio at the mixer and, therefore, the fluid of “altered” viscosity must displace all of the fluid and gel between the mixer and the outlet of the holding tank before the viscosity at the outlet of the holding tank is altered.
An apparatus and method for continuously hydrating a particulated polymer and producing a well treatment gel is described in U.S. Pat. No. 5,382,411 to Allen and is incorporated herein by reference for all purposes. In Allen, a mixer is employed to spray the polymer with water at a substantially constant water velocity and spray pattern at various rates of water flow. A centrifugal diffuser receives the mixture and passively converts the motion of the mixture thereby separating air from the mixture.